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Printed on 6/29/2026
For informational purposes only. This is not medical advice.
The Fisher grade classifies subarachnoid hemorrhage based on the amount and distribution of blood seen on initial CT scan. Grade 3 (thick clot) carries the highest vasospasm risk. It complements the Hunt & Hess clinical grading scale for comprehensive SAH assessment.
Formula: Grade 1: no blood. Grade 2: thin SAH <1mm. Grade 3: thick SAH ≥1mm. Grade 4: ICH/IVH ± SAH.
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Non-contrast CT is the first-line imaging study. It is most sensitive for detecting subarachnoid blood when performed within 6 hours of ictus (sensitivity ~98%). Identify the pattern, distribution, and density of subarachnoid blood in the basal cisterns, sylvian fissures, and sulci.
Grade I: no blood on CT (paradoxically some vasospasm risk). Grade II: diffuse thin SAH with no layer >1 mm thick. Grade III: thick cisternal clot or layer ≥1 mm — highest vasospasm risk (~23–37%). Grade IV: diffuse or thin SAH with intracerebral (ICH) or intraventricular hemorrhage (IVH).
Fisher Grade III carries the highest risk of delayed cerebral ischemia (DCI). Grade III patients warrant daily transcranial Doppler, close clinical monitoring from days 4–14, and aggressive vasospasm treatment protocols. The modified Fisher scale (Frontera 2006) incorporates IVH and is more commonly used in contemporary practice.
Neurointensivists, neurosurgeons
The Fisher grade directly estimates vasospasm risk from the initial CT, allowing stratification of patients into higher or lower surveillance intensity from day 1. Grade III warrants the most aggressive monitoring during the days 4–14 vasospasm window.
Neurocritical care teams, TCD technicians
Transcranial Doppler (TCD) is the primary non-invasive monitoring tool for vasospasm. Fisher Grade III patients require daily TCD starting around day 3–4, with increased frequency if velocities begin to rise. Grades I–II may still require monitoring but at lower intensity.
Neurointensivists, ICU nurses
Fisher grade informs institutional vasospasm surveillance protocols, determining neurological examination frequency, TCD schedule, and threshold for CT perfusion or CTA when clinical deterioration is suspected.
Neurosurgeons, neurologists, intensivists
Fisher grade is a standard component of SAH documentation and handoff communication. Combined with Hunt-Hess grade, it provides a comprehensive picture of clinical severity (clinical grade) and radiographic severity (CT grade) for team-based prognostication.
Neuroscience researchers, clinical trialists
Fisher grade is a required variable in SAH registries and clinical trials, enabling stratification for vasospasm risk and outcome analysis. It allows comparison of vasospasm rates and DCI across sites with different patient populations.
In the original Fisher classification, Grade I (no blood on CT) paradoxically showed ~21% vasospasm risk — higher than Grade II (~25%) but possibly reflecting cases where SAH was present but missed by CT, or where clinical/angiographic vasospasm was measured by different criteria. This has been better addressed by the modified Fisher scale.
The modified Fisher scale (mFisher) incorporates both cisternal blood thickness and IVH separately: 0 = no blood, 1 = thin diffuse SAH without IVH, 2 = thin diffuse SAH with IVH, 3 = thick cisternal SAH without IVH, 4 = thick cisternal SAH with IVH. Higher mFisher scores correlate better with DCI risk (AUC 0.71 vs 0.60 for original Fisher).
The mechanism of vasospasm involves oxyhemoglobin and other blood breakdown products from subarachnoid clot irritating the adventitia of adjacent arteries. Thick clot in the basal cisterns — where major intracranial vessels (M1, A1, basilar perforators) are concentrated — produces the most potent vasospastic stimulus. Fisher Grade III identifies this pattern.
DCI from vasospasm peaks around days 7–10 after SAH but can occur from days 4–14. Daily TCD monitoring of mean flow velocities is standard in most SAH centers. Mean velocity >120 cm/s in the MCA is a threshold for increased surveillance; >200 cm/s is considered severe vasospasm with high DCI risk.
The classic triple-H therapy (hypertension, hypervolemia, hemodilution) for symptomatic vasospasm is largely replaced by current practice favoring euvolemia combined with induced hypertension (systolic BP target 160–220 mmHg) for symptomatic DCI. Aggressive hypervolemia causes more complications than benefit; euvolemia with BP augmentation is safer.
Nimodipine is given to all SAH patients regardless of Fisher grade because its benefit operates through neuronal calcium channels, not vasodilation per se. It reduces DCI and poor outcomes by approximately 34%. It must be given orally or via NG tube — IV nimodipine is not approved in the US (only oral formulation).
Blood in the subarachnoid space begins to clear within hours. A CT performed 24–48 hours after hemorrhage may show less blood than was originally present, potentially downgrading the Fisher classification and underestimating vasospasm risk. For this reason, Fisher grade should ideally be assigned from the earliest CT available.
Angiographic vasospasm (vessel narrowing on CTA/DSA/TCD) without clinical symptoms is called silent or asymptomatic vasospasm. It occurs more commonly than symptomatic vasospasm. Symptomatic DCI — new focal deficit, altered consciousness — requires active treatment with induced hypertension and consideration of intra-arterial rescue therapy (verapamil, nimodipine intra-arterially, or balloon angioplasty).
Original Fisher scale by Fisher et al. (Neurosurgery 1980). Modified Fisher scale (Frontera et al., J Neurosurg 2006) showed better correlation with symptomatic vasospasm (AUC 0.71 vs 0.60). Delayed cerebral ischemia incidence is approximately 20–30% after SAH. Nimodipine evidence from Pickard et al. BMJ 1989. Current SAH vasospasm management guidelines from Connolly et al. (Stroke 2012).
Your Fisher grade classifies the amount and pattern of subarachnoid blood visible on the initial CT scan, which directly correlates with the risk of delayed cerebral vasospasm. Grade 1 (no blood detected on CT) carries the lowest vasospasm risk at approximately 21%. Grade 2 (diffuse thin layers of blood less than 1 mm thick) has a vasospasm risk of about 25%. Grade 3 (localized thick clot or layers of blood 1 mm or greater) carries the highest vasospasm risk at approximately 37%. Counterintuitively, Grade 4 (intracerebral or intraventricular hemorrhage with diffuse or absent subarachnoid blood) has a somewhat lower vasospasm risk (~31%) than Grade 3, though it may carry worse overall outcomes due to the parenchymal or ventricular blood.
Vasospasm typically occurs between days 3 and 14 after SAH, peaking around days 7–10. Your Fisher grade helps determine the intensity of vasospasm surveillance required during this critical window.
Use the Fisher grade when evaluating the initial non-contrast CT scan in a patient with confirmed subarachnoid hemorrhage. It should be assigned as soon as imaging is reviewed, ideally in conjunction with the Hunt & Hess clinical grade, to provide a comprehensive assessment of SAH severity and expected complications.
The Fisher grade is particularly valuable for guiding vasospasm prevention and monitoring strategies. Higher grades (especially Grade 3) warrant aggressive monitoring with transcranial Doppler ultrasonography, nimodipine prophylaxis, and potentially CT angiography if vasospasm is suspected. It is routinely used in neurosurgical and neurointensive care documentation and in research protocols studying SAH outcomes.
The original Fisher scale has been criticized for the paradoxical lower vasospasm risk in Grade 4 compared to Grade 3 and for its limited granularity. The Modified Fisher scale (Claassen et al., 2001) was developed to address these issues by separately scoring the presence of thick subarachnoid blood and intraventricular hemorrhage, providing better vasospasm prediction.
The Fisher grade depends on CT image quality and timing. Very early CT scans (within hours of ictus) may underestimate blood volume, while delayed scans may show blood redistribution or clearance. Inter-observer agreement is moderate, as the distinction between thin and thick subarachnoid blood (the critical 1 mm threshold) can be subjective. The scale also does not account for other vasospasm risk factors such as patient age, smoking history, hypertension, or clinical grade at presentation.
Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with questions about your health.
April 21, 2026 · trust-baseline
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